During recent years, the discharge of treated effluents has necessitated high wastewater treatment requirements at both the state and federal levels. Of particular concern is the increase in phosphates and nitrogen in the effluents which result in water bodies becoming enriched with plant nutrients causing a proliferation of water plants and algae which, in turn, result in various water purification and health problems. The effective removal of pollutants from wastewaters, particularly carbonaceous materials and nutrients such as nitrogen and phosphorus, has also become increasingly important in efforts to supplement and reuse existing municipal water resources.
Nitrogen and phosphorus concentrations in water sources have increased considerably due, in part, to chemical fertilizers which contain a substantial amount of nitrogen and phosphorus. This creates an increasing problem to public health because high levels of nitrate in drinking water can cause serious illness in human beings. As such, removal of substantial amounts of phosphorus and nitrate from water sources has become necessary to provide a clean municipal water supply.
The presence of organic materials in water also results in fungal and other heterotrophic growths and deoxidation of the water (due to the metabolic activities of the growths) which render the water unsuitable for higher life forms such as fish. These aerobic conditions also cause fermentation and redissolution of heavy metal salts. These net effects reduce the aesthetic appearance, recreational use and reuse of the water.
In the past, a wide range of physical, chemical and biological processes have been proposed for eliminating pollutants in existing wastewater streams, particularly carbonaceous material and unwanted nutrients. Other efforts have focused on the reduction of total suspended solids in treated effluents. The so-called "biological" nutrient removal processes have been found particularly attractive for the treatment of municipal wastewaters since such treatment costs have generally been less than the cost associated with physical-chemical methods, and the characteristics of the wastewater are amenable to biological treatment.
In that regard, it has long been known that the quality of effluent may be improved under aerobic conditions in which bacteria metabolize the biodegradable organics, using dissolved oxygen as the terminal electron acceptor. Approximately one-third of the metabolized organics are oxidized to carbon dioxide and water to obtain the energy to convert the remaining two-thirds of the organics to microbial protoplasm. The major problem in basic aerobic treatment, however, is the enlarged volume of microbial solids to be processed.
In anaerobic systems, i.e., those operated in the absence of introduced oxygen, either elemental or derived from nitrates or nitrites, the bacteria must find another electronic acceptor. In anoxic systems, i.e., oxygen absent but nitrate present, chemically bound oxygen becomes the primary electronic acceptor such as, when nitrates are reduced to nitrites and various intermediates before being reduced to nitrogen gas. The denitrification results in the eventual production of nitrogen gas, which is insoluble in the wastewater, rather than in the production of ammonia or ammonium ions.
In order to remove phosphorus from wastewater, anaerobic conditions are utilized. This allows microorganisms to hydrolyze polyphosphate stored during prior oxidation processes, thereby releasing soluble phosphate to the mixed liquor in the anaerobic zone.
Because of the need to reduce the level of organic pollutants, as well as nutrients such as nitrogen and phosphorus, many conventional "biological" waste treatment systems have combined aerobic and anaerobic steps, generally with the anaerobic stage being the initial treatment step. One principal difficulty encountered with such combined processes is that time becomes a critical variable in designing and sizing the wastewater treatment system. A different time period is required to metabolize a given amount of organic matter by a unit of cell mass. By retaining the microbes in the treatment system, the treatment time per unit of organic matter is reduced. Because the time for aerobic treatment is controlled by oxygen transfer, contact between the microbes and the organic pollutants controls the total reaction time.
Unfortunately, the kinetics of nitrogen and phosphorus removal under anaerobic conditions are not always compatible with the treatment and removal of organic materials in an aerobic environment. For example, the rate of removal of phosphorus is a direct function of the concentration of organics. Thus, it is important to maintain higher concentrations of residual organics at a point during the process when phosphorus uptake rates can be optimized. As a result, the design of systems capable of using combined treatment mechanisms (aerobic, anaerobic and anoxic) have often been unduly complicated and expensive to install and operate.
During recent years, the activated sludge process has proven to be an effective means for the removal of biological oxygen demand (BOD) from wastewater and for producing high quality effluent with reduced total suspended solids concentrations. The process has been extensively described in the literature and, in general, includes the step of maintaining an aeration system in which the wastewater is fed to a suspension of microorganisms which are responsible for removing excess bacteria and producing a clarified effluent.
Because bacteria cannot metabolize solid organics, they convert certain solid particles to soluble organics prior to metabolism by virtue of enzymes in the cell surface capable of hydrolyzing the complex organics to simple organic molecules. It is known that the bacteria must have a suitable environment with all the proper nutrients. The environment must also provide good mixing for adequate contact between the microorganisms and the pollutants being metabolized and may involve an aerobic system with excess dissolved oxygen or an anoxic or anaerobic system without dissolved oxygen. In either case, the known activated sludge processes require sufficient nitrogen, phosphorus, iron and trace metals for good growth of the microorganisms, without high concentrations of heavy metals.
Typical wastewater treatment processes usually include multiple treatment areas or zones which can be roughly broken down into: (1) a preliminary treatment area; (2) a primary treatment area; and (3) a secondary treatment area. The wastewater treatment process begins with the preliminary treatment area. Preliminary treatment is concerned with removing grit and damaging debris, such as cans, bath towels, etc., from the untreated wastewater. This is usually a two-stage treatment process whereby the debris such as rags and cans are moved by screens and the grit and heavy inorganic solids settle out of the untreated wastewater as it passes through a velocity controlled zone. The damaging inorganic debris is thus removed by screening or settling while organic matter carried within the fluid stream passes on.
Following the preliminary treatment area, the wastewater is directed to a primary treatment area. The primary treatment area entails a physical process wherein a portion of the organics is removed by flotation or sedimentation. The organics removed include feces, food particles, grease, paper, etc., and are technically defined as suspended solids. Usually 40 to 70% of the suspended solids are removed in this primary stage. The third treatment stage is called secondary treatment and is usually a biological treatment process where bacteria are utilized under controlled conditions to remove nutrients or nonsettling suspended and soluble organics from the wastewater. These materials would result in an unacceptable biological oxygen demand (BOD) if left untreated. Typically, one mode of this process consists of a basin in which the wastewater is mixed with a suspension of microorganisms. This mixture is then aerated to provide oxygen for support of the microorganisms which may then absorb, assimilate, and metabolize the excess biological oxygen demand in the wastewater. After sufficient retention time, the mixture is then introduced into a clarifier or settler into which the biomass separates as settled sludge from the liquid. The purified fluid then overflows into a receiving stream.
There are three principal types of secondary treatment for affecting treatment of wastewater. The first type, known as a trickling filter, allows the wastewater to trickle down through a bed of stone whereby the organic material present in the wastewater is oxidized by the action of microorganisms attached to the stone. A similar concept is the RBC, or the rotating biological contactor, wherein the biology is attached to the media which rotates in the wastewater and purifies it in the manner of a trickling filter. The second method is an activated sludge process in which the wastewater is fully aerated and agitated by either compressed air or mechanical means together with a portion of the biomass or activated sludge which has been returned from the clarifier or settler. The third process may be referred to as a semi-aerobic (anaerobic/aerobic) process in which the first stage is anaerobic or anoxic, followed by an aerobic stage. This anaerobic-anoxic-aerobic process is very similar to the initial stages of the Phoredox process and the modified Bardenpho process, known well in the wastewater treatment industry.
This anaerobic-aerobic process was first disclosed in U.S. Pat. Nos. 2,788,127 and 2,875,151 by Davidson. In the anaerobic-aerobic process, the untreated wastewater is first subjected to anaerobic treatment and then to aerobic decomposition. A portion of the sludge formed during the aerobic decomposition is recycled back and mixed with the untreated wastewater being subjected to anaerobic treatment. Davidson noted that the aerobic organisms in the recycled activated sludge are not impaired by passage through the anaerobic reactor and may, in fact, undergo unusual stimulation.
In recent years, there has been a great deal of work directed at biological processes for removing pollutants such as phosphorus and nitrogen from wastewater. This work has in large part been broadly based and has not been focused on specific problems and concerns. For example, many wastewater facilities are now facing very stringent phosphorus control standards. When there is already a wastewater treatment facility in place, it becomes prudent to consider the possibility of modifying these existing facilities in order to meet new standards being imposed. Obviously costs, both initial and operating, are of main concern. One important concern then is to evaluate the economics of modifying existing treatment facilities to accomplish biological phosphorus and nitrogen removal.
In U.S. Pat. No. 4,056,465 a modified activated sludge system is disclosed wherein BOD-containing wastewater and recycled sludge are initially admixed under anaerobic conditions in the substantial absence of oxygen or oxidizing agents and subsequently subjected to aeration and clarification. Nitrates and nitrites are removed by interposing an anoxic treating zone between the anaerobic zone and the aerating zone. The patent suggests that the initial admixture of the recycled sludge or biomass with the wastewater influent be under anaerobic conditions such that the basin or zone in which the mixed liquor is first formed is substantially free of nitrites or nitrates and dissolved oxygen.
In U.S. Pat. No. 4,271,026 there is described a wastewater treatment process for enhanced phosphorus removal at adequately high rate process operations. This is accomplished by maintaining a particular set of interrelated operating conditions within a specific envelope in the type of process where recycled activated sludge is mixed with a wastewater influent containing phosphate and BOD under anaerobic conditions, thereby promoting selective production of the desired type of microorganism.
In U.S. Pat. No. 4,488,968 a treatment of wastewater is described in which the wastewater influent is initially mixed with recycle active sludge in an anaerobic zone and then subjected to aeration in an aerobic zone, wherein the residence time of the mixed liquor in the aerobic zone is reduced. At least part of the sludge separated from the mixed liquor is subjected to further oxidation in a separate zone before admixture with the wastewater influent.
The process disclosed in U.S. Pat. No. 4,948,510 employs a plurality of basins which may be individually controlled to achieve anaerobic, anoxic or aerobic conditions. The basins are reconfigurable in that the flow of effluent to a basin, transfer of mix liquor between basins and effluent discharge from a basin can be varied to create a treatment cycle which has features of both continuous and batch processes while minimizing recycle rates and hydraulic level changes.
Other return activated sludge wastewater treatment processes have been disclosed which utilize various anoxic and aeration zones or cells to biologically remove phosphorus and nitrogen. For example, in U.S. Pat. No. 4,867,883 there is described a wastewater treatment process wherein the return sludge is denitrified by the mixed liquor suspended solids (MLSS) from a preceding anaerobic zone which receives an internal recycle from an anoxic zone. In U.S. Pat. No. 4,999,111 there is described a wastewater treatment process in which the return sludge is pretreated (nutrified) by unaerated contact of fermentation liquids produced from primary sludge. This contact is completed in one or more stages and the initial stage may be anoxic or anaerobic depending on the nitrate content of the return sludge. Additionally, in U.S. Pat. No. 4,956,094, a wastewater treatment process for the removal of phosphorus is described. This process involves the addition of carbonaceous oxygen demand (COD) or BOD containing liquors to a portion of the return activated sludge (RAS) which is mixed anaerobically and then settled to separate released soluble phosphates and the solids. The anaerobic mix zone consists of a pre-stripper followed by the settling unit with soluble phosphorus removed from the supernatant liquid by chemical precipitation.
Another method for removing nitrogen from wastewater includes the step feed activated sludge process. As described in a technical paper by Kaiser et al., the step feed process has been successful as applied to nitrification-denitrification wastewater treatment processes. Although some biological phosphorus removal was observed, performance was inconsistent. Therefore, the use of a step feed process to achieve consistent simultaneous biological nitrogen and phosphorus removal has not been discovered.
There have been attempts at biologically controlling nitrogen and phosphorus by incorporating an anoxic zone downstream from a series of preceding zones that would typically include aerobic and anaerobic zones. In order to remove and control those pollutants traditionally considered of prime importance, such as ammonia nitrogen, BOD, and phosphorus, these biological processes require that the anaerobic and aerobic zones be disposed in initial stages of treatment. For example, anaerobic zones, necessary for biological phosphorus removal, deplete microorganism food source, i.e., BOD. Consequently, by the time the wastewater mixed liquor has reached the downstream anoxic zone, there is very little, if any, food source for the microorganisms. Without food, the effectiveness of downstream denitrification is seriously hampered and usually inefficient. Besides that, the overall effectiveness of such a biological denitrification/dephosphorization process depends on flow in the overall makeup of the wastewater which can vary sharply from time to time.
Additionally, upstream aerobic zones which are required for the removal of nitrogen produce nitrites and nitrates which interfere with the efficient development of anaerobic conditions. Accordingly, previous phosphorus and nitrogen wastewater removal processes have not been able to consistently and adequately remove sufficient quantities of phosphorus and nitrogen simultaneously.
Therefore, there is a need for a biological denitrification/dephosphorization process which integrates anaerobic and aerobic zones to provide efficient removal of phosphorus and nitrogen. Additionally, there is a need for the application of such a process to existing wastewater treatment facilities without the need for additional tankage. Moreover, there is a need for a wastewater treatment process which can be retrofitted more easily onto small wastewater treatment plant sites having reduced tankage facilities.